John Posey played Danny Tanner in the unaired Full House pilot included on the Season 1 DVD.
The pilot was produced before Bob Saget could take the role. Saget was unavailable because of his work as an on-air contributor to CBS’s The Morning Program, so the producers hired Posey to portray Danny, the widowed father raising three daughters with help from Jesse and Joey.
Posey appeared only in that unaired pilot. After Saget became available, the production replaced Posey, and Saget became the Danny Tanner audiences know from the broadcast series. The pilot was not part of the original ABC television run, but Warner Home Video included it as a bonus on the DVD release of the first season.
This makes the question easy to confuse with the show’s regular casting. Richard Karn, Tom Hanks, and Alan Thicke were not the pilot’s Danny. Posey’s performance is significant precisely because it preserves an alternate version of the show before the familiar cast and presentation were finalized.