Sitting in a starship captain’s chair is a big job. Making a documentary about the actors whose posteriors have occupied that hallowed space through four decades of “Star Trek” and its spin-offs takes a big man. Thank the Vulcan gods that William Shatner was available. In “The Captains,” which Mr. Shatner wrote and directed, he interviews the five performers who have succeeded him as “Star Trek” leaders, on starships or space stations: Patrick Stewart, Avery Brooks, Kate Mulgrew, Scott Bakula and Chris Pine. The film will make its debut on Friday on the Epix premium-cable channel and epixhd . com as part of Shatnerpalooza, a conglomeration of television marathon, film screenings and live appearances.