And I just realised...

She was in Captain America, and on Doctor Who. (Guess who)

I think this is because movies are permanent. What happened in them happened and therefore if you were in it, that's it, you were in it. But a show has different episodes and seasons and when someone appears they probably won't be 'in' every single one. Like guest stars on Sesame Street. Katy Perry was 'on' Sesame Street.

Cuz of "ON air" and "IN theaters" hahaha

I think because when you're talking about a movie you're just talking about one movies... Whereas when you are talking about a TV show there's multiple episodes.