The Costume timeline is the first timeline (teal colored horizontal bar) in the Keyframer and dictates the appearance of the Actor, i.e, what costume the Actor is wearing. If a Costume timeline is present the Actor is on stage and is performing some role.
Note: an Actor can exist without a Costume timeline, but it will be something like the Invisible Man. You won't see it in the Animator until it puts on a Costume. This is an effective way to make Actors disappear during animations; just delete their Costume timeline from the appropriate frames.
With the exception of Camera and Target type Actors, which have no detailed Costume parameters, settings are grouped in a dialog which appears when a timeline is created. The dialog shows the timeline extent, and other settings which will depend on the type of Actor.
Double clicking on the keyframe indicator at the end of a costume timeline shows a dialog box, with buttons to edit the settings for the Costume, delete it, cut it out, or copy it.
Edit - Brings up the Edit Settings dialog for the Actor. In the above example, Costume for the Model Actor, the Edit Model Dialog would appear.
Cancel - Dismisses the Costume timeline dialog.
Delete - Deleting a timeline removes it from the Keyframer.
Cut Out - Cutting out a costume timeline deletes the timeline and shifts all later movement timelines back in time to fill the gap.
Copy - Copying a costume timeline places a new timeline with the same settings and duration at the end of all the other costume timelines for this Actor. You can use this feature to 'leapfrog' costume copies in a repeating cycle for Morph or Robot Actor sequences.