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She was somewhat known at the time, having starred in several Disney movies before
Taxi Driver
. The studio probably boxed her name to highlight the fact that a popular Disney kid is now in an R-rated movie. In her case, they probably did it to help attract controversy (and, as a result, audiences).
You mustn't be afraid to dream a little bigger, darling.

Jodie Foster wasn't a popular actor, before Taxi Driver.
Carpe Noctem!

It's not boxed but Tom Wilkinson gets an "And" credit in pretty much every damned movie he does these days.
"I shall tread uncommon wary and keep my pepperbox handy."

I was wondering the same thing that is, why some actors in some films received a box around their name. His name is also boxed on the poster, the DVD cover, and in the film's credits. It never had anything to do with the actor's death, since Christopher Walken received the same treatment for
Batman Returns
, as did Kim Basinger for
Batman
. Other examples include John Lithgow for
Footloose
, Tony Randall in
The King of Comedy
, Jodie Foster and Peter Boyle in
Taxi Driver
, and many others. It seems to just be a way to draw attention to a popular actor who has a supporting role in a film. They don't receive top billing, so they want to make sure their name sticks out among the rest of the credits. This practice seems to have fallen into disuse in recent years.
That's a
BINGO!

Makes sense. Must have confused it with something else. Thanks.

Isn't it a conventional way to flag up the presence of a famous actor in a small-ish supporting part? I think David Warner's name was in a rectangle on the posters for The Leading Man, and I'm sure I've seen other examples of this (can't think of the movies offhand though).