diff --git a/decoy/__init__.py b/decoy/__init__.py
index eea7e12..06641d6 100644
--- a/decoy/__init__.py
+++ b/decoy/__init__.py
@@ -159,15 +159,16 @@ def reset(self) -> None:
 
 
 class Stub(Generic[ReturnT]):
-    """A rehearsed Stub that can be used to configure mock behaviors."""
+    """A rehearsed Stub that can be used to configure mock behaviors.
+
+    See [stubbing usage guide](../usage/when) for more details.
+    """
 
     def __init__(self, core: StubCore) -> None:
         self._core = core
 
     def then_return(self, *values: ReturnT) -> None:
-        """Set the stub to return value(s).
-
-        See [stubbing usage guide](../usage/when) for more details.
+        """Configure the stub to return value(s).
 
         Arguments:
             *values: Zero or more return values. Multiple values will result
@@ -177,9 +178,7 @@ def then_return(self, *values: ReturnT) -> None:
         self._core.then_return(*values)
 
     def then_raise(self, error: Exception) -> None:
-        """Set the stub to raise an error.
-
-        See [stubbing usage guide](../usage/when) for more details.
+        """Configure the stub to raise an error.
 
         Arguments:
             error: The error to raise.
@@ -191,5 +190,14 @@ def then_raise(self, error: Exception) -> None:
         """
         self._core.then_raise(error)
 
+    def then_do(self, action: Callable[..., ReturnT]) -> None:
+        """Configure the stub to trigger an action.
+
+        Arguments:
+            action: The function to call. Called with whatever arguments
+                are actually passed to the stub.
+        """
+        self._core.then_do(action)
+
 
 __all__ = ["Decoy", "Stub", "matchers", "warnings", "errors"]
diff --git a/decoy/call_handler.py b/decoy/call_handler.py
index e0e8939..cbf72ca 100644
--- a/decoy/call_handler.py
+++ b/decoy/call_handler.py
@@ -22,4 +22,7 @@ def handle(self, call: SpyCall) -> Any:
         if behavior.error:
             raise behavior.error
 
+        if behavior.action:
+            return behavior.action(*call.args, **call.kwargs)
+
         return behavior.return_value
diff --git a/decoy/core.py b/decoy/core.py
index 8c3a7a7..76fff8e 100644
--- a/decoy/core.py
+++ b/decoy/core.py
@@ -1,6 +1,6 @@
 """Decoy implementation logic."""
 from __future__ import annotations
-from typing import Any, Optional
+from typing import Any, Callable, Optional
 
 from .spy import SpyConfig, SpyFactory, create_spy as default_create_spy
 from .spy_calls import WhenRehearsal
@@ -89,3 +89,10 @@ def then_raise(self, error: Exception) -> None:
             rehearsal=self._rehearsal,
             behavior=StubBehavior(error=error),
         )
+
+    def then_do(self, action: Callable[..., ReturnT]) -> None:
+        """Set the stub to perform an action."""
+        self._stub_store.add(
+            rehearsal=self._rehearsal,
+            behavior=StubBehavior(action=action),
+        )
diff --git a/decoy/stub_store.py b/decoy/stub_store.py
index be1ba6a..e11f515 100644
--- a/decoy/stub_store.py
+++ b/decoy/stub_store.py
@@ -1,5 +1,5 @@
 """Stub creation and storage."""
-from typing import Any, List, NamedTuple, Optional
+from typing import Any, Callable, List, NamedTuple, Optional
 
 from .spy_calls import SpyCall, WhenRehearsal
 
@@ -9,6 +9,7 @@ class StubBehavior(NamedTuple):
 
     return_value: Optional[Any] = None
     error: Optional[Exception] = None
+    action: Optional[Callable[..., Any]] = None
     once: bool = False
 
 
diff --git a/docs/usage/when.md b/docs/usage/when.md
index 18cc766..a6c6028 100644
--- a/docs/usage/when.md
+++ b/docs/usage/when.md
@@ -2,45 +2,114 @@
 
 A stub is a mock that is pre-configured to return a result or raise an error if called according to a specification. In Decoy, you use [decoy.Decoy.when][] to configure stubs.
 
-## Using rehearsals to return a value
+## Configuring a stub
 
-`Decoy.when` uses a "rehearsal" syntax to configure a stub's conditions:
+`Decoy.when` uses a "rehearsal" syntax to configure a stub's conditions. To configure a stubbed behavior, form the expected call to the mock and wrap it in `when`:
 
 ```python
 def test_my_thing(decoy: Decoy) -> None:
     database = decoy.mock(cls=Database)
     subject = MyThing(database=database)
 
-    decoy.when(database.get("some-id")).then_return(Model(id="some-id"))
-
-    result = subject.get_model_by_id("some-id")
-
-    assert result == Model(id="some-id")
+    stub = decoy.when(
+        database.get("some-id")  # <-- rehearsal
+    )
+    ...
 ```
 
-The "rehearsal" is simply a call to the stub wrapped inside `decoy.when`. Decoy is able to differentiate between rehearsal calls and actual calls. If the mock is called later **in exactly the same way as a rehearsal**, it will behave as configured. If you need to loosen the "exact argument match" behavior, see [matchers](./matchers).
+Any time your dependency is called **in exactly the same way as the rehearsal**, whatever stub behaviors you configure will be triggered. If you need to loosen the "exact argument match" behavior, you can use [matchers](./matchers).
 
 The "rehearsal" API gives us the following benefits:
 
--   Your test double will only return the value **if it is called correctly**
+-   Your test double will only take action **if it is called correctly**
     -   Therefore, you avoid separate "configure return" and "assert called" steps
 -   If you use type annotations, you get typechecking for free
 
+## Returning a value
+
+To configure a return value, use [decoy.Stub.then_return][]:
+
+```python
+def test_my_thing(decoy: Decoy) -> None:
+    database = decoy.mock(cls=Database)
+    subject = MyThing(database=database)
+
+    decoy.when(
+        database.get("some-id")  # <-- when `database.get` is called with "some-id"
+    ).then_return(
+        Model(id="some-id")  # <-- then return the value `Model(id="some-id")`
+    )
+
+    result = subject.get_model_by_id("some-id")
+
+    assert result == Model(id="some-id")
+```
+
+The value that you pass to `then_return` will be type-checked.
+
 ## Raising an error
 
-You can configure your stub to raise an error if called in a certain way:
+To configure a raised exception when called, use [decoy.Stub.then_raise][]:
 
 ```python
 def test_my_thing_when_database_raises(decoy: Decoy) -> None:
     database = decoy.mock(cls=Database)
     subject = MyThing(database=database)
 
-    decoy.when(database.get("foo")).then_raise(KeyError(f"foo does not exist"))
+    decoy.when(
+        database.get("foo")  # <-- when `database.get` is called with "foo"
+    ).then_raise(
+        KeyError("foo does not exist")  # <-- then raise a KeyError
+    )
 
     with pytest.raises(KeyError):
         subject.get_model_by_id("foo")
 ```
 
+**Note:** configuring a stub to raise will **make future rehearsals with the same arguments raise.** If you must configure a new behavior after a raise, use a `try/except` block:
+
+```python
+decoy.when(database.get("foo")).then_raise(KeyError("oh no"))
+
+# ...later
+
+try:
+    database.get("foo")
+except Exception:
+    pass
+finally:
+    # even though `database.get` is not inside the `when`, Decoy
+    # will pop the last call off its stack to use as the rehearsal
+    decoy.when().then_return("hurray!")
+```
+
+## Performing an action
+
+For complex situations, you may find that you want your stub to trigger a side-effect when called. For this, use [decoy.Stub.then_do][].
+
+This is a powerful feature, and if you find yourself reaching for it, you should first consider if your code under test can be reorganized to be tested in a more straightforward manner.
+
+```python
+def test_my_thing_when_database_raises(decoy: Decoy) -> None:
+    database = decoy.mock(cls=Database)
+    subject = MyThing(database=database)
+
+    def _side_effect(key):
+        print(f"Getting {key}")
+        return Model(id=key)
+
+    decoy.when(
+        database.get("foo") # <-- when `database.get` is called with "foo"
+    ).then_do(
+        _side_effect # <-- then run `_side_effect`
+    )
+
+    with pytest.raises(KeyError):
+        subject.get_model_by_id("foo")
+```
+
+The action function passed to `then_do` will be passed any arguments given to the stub, and the stub will return whatever value is returned by the action.
+
 ## Stubbing with async/await
 
 If your dependency uses async/await, simply add `await` to the rehearsal:
@@ -51,7 +120,11 @@ async def test_my_async_thing(decoy: Decoy) -> None:
     database = decoy.mock(cls=Database)
     subject = MyThing(database=database)
 
-    decoy.when(await database.get("some-id")).then_return(Model(id="some-id"))
+    decoy.when(
+        await database.get("some-id")  # <-- when database.get(...) is awaited
+    ).then_return(
+        Model(id="some-id")  # <-- then return a value
+    )
 
     result = await subject.get_model_by_id("some-id")
 
diff --git a/tests/test_call_handler.py b/tests/test_call_handler.py
index 612c89e..c70298c 100644
--- a/tests/test_call_handler.py
+++ b/tests/test_call_handler.py
@@ -75,7 +75,7 @@ def test_handle_call_with_raise(
     stub_store: StubStore,
     subject: CallHandler,
 ) -> None:
-    """It return a Stub's configured return value."""
+    """It raise a Stub's configured error."""
     spy_call = SpyCall(spy_id=42, spy_name="spy_name", args=(), kwargs={})
     behavior = StubBehavior(error=RuntimeError("oh no"))
 
@@ -85,3 +85,22 @@ def test_handle_call_with_raise(
         subject.handle(spy_call)
 
     decoy.verify(call_stack.push(spy_call))
+
+
+def test_handle_call_with_action(
+    decoy: Decoy,
+    call_stack: CallStack,
+    stub_store: StubStore,
+    subject: CallHandler,
+) -> None:
+    """It should trigger a stub's configured action."""
+    action = decoy.mock()
+    spy_call = SpyCall(spy_id=42, spy_name="spy_name", args=(1,), kwargs={"foo": "bar"})
+    behavior = StubBehavior(action=action)
+
+    decoy.when(stub_store.get_by_call(spy_call)).then_return(behavior)
+    decoy.when(action(1, foo="bar")).then_return("hello world")
+
+    result = subject.handle(spy_call)
+
+    assert result == "hello world"
diff --git a/tests/test_core.py b/tests/test_core.py
index b877ef5..dc6ceeb 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -175,6 +175,28 @@ def test_when_then_raise(
     )
 
 
+def test_when_then_do(
+    decoy: Decoy,
+    call_stack: CallStack,
+    stub_store: StubStore,
+    subject: DecoyCore,
+) -> None:
+    """It should add an action behavior to a stub."""
+    rehearsal = WhenRehearsal(spy_id=1, spy_name="my_spy", args=(), kwargs={})
+    decoy.when(call_stack.consume_when_rehearsal()).then_return(rehearsal)
+
+    action = lambda: "hello world"  # noqa: E731
+    result = subject.when("__rehearsal__")
+    result.then_do(action)
+
+    decoy.verify(
+        stub_store.add(
+            rehearsal=rehearsal,
+            behavior=StubBehavior(action=action),
+        )
+    )
+
+
 def test_verify(
     decoy: Decoy,
     call_stack: CallStack,
diff --git a/tests/test_decoy.py b/tests/test_decoy.py
index 5b2b590..9b03bb1 100644
--- a/tests/test_decoy.py
+++ b/tests/test_decoy.py
@@ -50,11 +50,28 @@ def test_when_smoke_test(decoy: Decoy) -> None:
     subject = decoy.mock(func=some_func)
 
     decoy.when(subject("hello")).then_return("hello world")
+    decoy.when(subject("goodbye")).then_raise(ValueError("oh no"))
+
+    action_result = None
+
+    def _then_do_action(arg: str) -> str:
+        nonlocal action_result
+        action_result = arg
+        return "hello from the other side"
+
+    decoy.when(subject("what's up")).then_do(_then_do_action)
 
     result = subject("hello")
     assert result == "hello world"
 
-    result = subject("goodbye")
+    with pytest.raises(ValueError, match="oh no"):
+        subject("goodbye")
+
+    result = subject("what's up")
+    assert action_result == "what's up"
+    assert result == "hello from the other side"
+
+    result = subject("asdfghjkl")
     assert result is None