Occasionally it becomes necessary to introduce or recall characters into the action because of real-world interruptions like starting a new game session and suddenly having more players in the game. This is why it's good to pause games at convenient stopping points in between adventure scenes, such as resting at an inn, or during time off work, or between interstellar delivery runs.
Sometimes, however, you're stuck and must break a session during a combat scene, or while chasing a criminal through the city streets, or while in hyperspace en route to a new planet. That can make things slightly trickier.
This is when you bring out the big guns: "Actually, the martial artist was with you all along! He just didn't do anything noteworthy until now."
R2-D2: All right. Massive fight scene time.
GM: Actually, it's getting late. We really don't want to start a combat now. Let's leave it until next session.
R2-D2: And yeah, Jim should be here for this.
GM: It could be a little difficult justifying Padmé appearing here.
Anakin: It would be good to see him, but I doubt he'll show up.
R2-D2: Look, I'll talk to him and make sure he's here.
Anakin: No, that won't help. I need to talk to him.
R2-D2: I insist.
Obi-Wan: Pete, I think you should drop it.
R2-D2: Next session when he turns up, you'll know who to thank.
Anakin: Stop trying to help!
Obi-Wan: Wow. I'd never considered shutting Pete up by accusing him of being too helpful.