The micro-budgeted horror film Insidious is aptly named, given the stealthy way it has become a box office smash. The paranormal tale, which stars Patrick Wilson and Rose Byrne as parents fighting for the soul of their comatose child, opened April 1 and grossed an impressive $13 million in its debut weekend. The film has held on strong, and last weekend it broke the $50 million barrier. That's a huge figure for a movie that cost ''a tiny bit north of $1 million,'' according to producer Jason Blum (Paranormal Activity). ''I felt we had something special. But I didn't imagine it would do so well.'' Director James Wan (Saw) credits good word of mouth and a strong pre-release campaign by newbie distributor FilmDistrict (which is also enjoying success with the low-budget Soul Surfer: $36.9 million and counting). ''They had screenings all over the country,'' explains Wan. The director says he has no plans for a sequel, but ''there's always a 'maybe' in Hollywood, right?''