Speculations about Radcliffe's involvement in the movie started after he was photographed with a dozen of dogs near Bryant Park in New York, reported Aceshowbiz.
No source has confirmed Radcliffe's casting and Apatow also played coy when a fan asked him whether or not the actor would appear in the film.
"Dan to cameo in Trainwreck?" the fan asked, to which the director jokingly replied, "Not true! He was just walking his dogs."
The actor was also seen filming a scene with 'Crazy, Stupid, Love' star Marisa Tomei.
The plot is kept under wraps, but Universal has announced that the film is slated for a July 2015 release.