A/the or no article?

Use 'the'. You need the definite article

Ok, but I wonder why. Is there any reason for using this one?

I'm not in complete agreement with Roger, I think using no article is also acceptable, but I'm with him on not using the indefinite article. Robert De Niro is famous, if it's not the Robert De Niro it's some bozo with a similar name. If you were talking about someone relatively unknown, the indefinite article would be appropriate, but not for someone of De Niro's stature.

I think you can also use the indefinite article. It means that, within the category of "American actors," you met one among them, Robert DeNiro.