NEW YORK—It was just another job for child actor Karolyn Grimes. She was 6 and had already done four movies by the summer of 1946 when filming began on “It’s a Wonderful Life.” The only thing she recalls about the biggest movie of her career was being delighted to play in snow on the set. That single job would become very memorable to a lot of other people. “It’s a Wonderful Life,” which marks its 75th anniversary this year, is now a beloved holiday tradition across the globe. “It’s a once-a-year pick-me-up for the humanity of America and the world,” Grimes told The Associated Press. “It’s a good impetus to really keep us going in a positive direction.” “It’s a Wonderful Life” was released in late 1946, produced and directed by Frank Capra and starring Jimmy Stewart and Donna Reed. It takes place on Christmas Eve in a small town. …