NodeEditor.OnBodyGUI

public override void OnBodyGUI();

Override to draw custom GUI in the node body. Code is very similar to writing custom inspectors. More info can be found here

[CustomNodeEditor(typeof(MyNodeEditor))]
public class MyNodeEditor : NodeEditor {

    public override void OnBodyGUI() {
        EditorGUILayout.LabelField("Text above");
        base.OnBodyGUI();
        EditorGUILayout.LabelField("Text below");
    }
}

See also: GetInputValue GetInputValues