If someone was a lead actor in a major Hollywood movie and the movie turned out successful and a blockbuster hit would they be a celeb??
Yes they would be.
For a time. Whether they stayed a celebrity beyond the celebrity of that movie would depend on them either getting another acting gig that keeps them in the limelight or fading into obscurity like some one-hit-wonder.
It depends really. Of course they could get their standard '15 minutes' but that impact can only last so long. Think of people like Macaulay Culkin, Jake Lloyd, Edward Furlong, and Tom Green just to name a few.
How would they not be?