Yeah, "elokuvassa" is singular, "elokuvissa" is plural. "At the movies" and "elokuvissa" mean the same. So if we have no context we can say it means she is at a movie theater. If you want to say that she acted in movies you could just say "Liisa näyttelee elokuvissa" or "Hän on tässä elokuvassa" (She is in this movie) etc.
You can also be "at the movies" without watching a film - although, like being "at the movie theater" and not watching, it would be weird and not what the listeners assumes.
If you want to be specific that she is in a theater watching right now, you could say "Lisa is out watching a movie"
I'll just clarify that the names don't come from English, they just have the same roots (like so many other names). Liisa is shortened from Elisabet, which derives from Hebrew Eliseba. And Rosa and Roosa are both Finnish names, and come from Latin Rosa (meaning rose).