Leaders of the Screen Actors Guild said on Tuesday evening that they had agreed to open formal contract talks with representatives of Hollywood's movie and television companies on April 15. The actors' current contract with the companies expires June 30, and much of the entertainment industry, hoping to avoid a strike similar to a three-month walkout by writers that began last November, has been eager to see the bargaining begin. The actors guild has about 120,000 members. Last weekend, the American Federation of Television and Radio Artists, which has bargained jointly with the guild in the past, broke off their alliance and said it, too, expected soon to begin negotiating a new contract on its own.