In contrast to his first season, when he made frequent substitutions during exhibition games, Coach Joe Gibbs says he will give many of his veteran Redskins considerable playing time tonight in their preseason opener against the Dolphins in Miami.

"I've modified my approach some," Gibbs said yesterday, talking about plans for the 8 o'clock game in the Orange Bowl (WJLA-TV-7). "Looking back, I didn't feel that some of the veterans got enough time in these games to be as sharp as they should have been by the start of the regular season.

"We pulled guys like John Riggins very early in the games. I want to give him more time. I also think that we didn't let the defensive line stay in long enough to develop real good game reactions. They need to go a little longer to get all their reads down."

Of course, the defensive line was one of the Redskins' problem areas entering training camp. Torgy Torgeson, who coaches the front four, said the staff will use the first two exhibitions to take a long look at those players. After that, he said, the coaches will make some decisions.

"If we see the line is progressing, especially the ends, then fine," Torgeson said. "If we see that we are still having major problems, then we'll have to adjust our thinking. Maybe we will do more shifting, things like that, to help out the players . . . We certainly aren't going to be stubborn and say we are staying with our early plans no matter how anyone is playing. And we need to make decisions early enough in camp to get things coordinated.

"The 3-4? That's always a possibility, too. It's in our playbook . . . "

Gibbs and his staff have kept their focus on the game, and not on the pregame gesture of union solidarity planned by players from both teams. Gibbs says he is "not concerned" about the midfield handshake the players plan, nor has he talked to his players about it.

In contrast, Dolphins Coach Don Shula has criticized the union's plans, although he apparently won't try to stop them. But that is certainly not the only contrast between the two coaches.

Gibbs wants to win, but also wants to give prospects at tight end (Clint Didier, Mike Williams), linebacker (Jeff Goff, Lemont Jeffers, Mel Kaufman, Larry Kubin, Quentin Lowry), wide receiver (Charlie Brown, Carl Powell, Rodney Goosby) and cornerback (Vernon Dean, LeCharls McDaniel) ample playing time so the players at those positions can be evaluated.

Shula always has been intent on winning. He plays veterans as much in preseason games as any NFL coach, and his success is reflected in his exhibition record at Miami (46-17-2); he has won 15 of his last 16 preseason contests.

"They are going to be very tough, but I think that's good for us," Gibbs said. "It will let us see how we play against a top-flight team using their best players for long periods . . ."

Unlike previous years, when the Dolphins were going through extensive rebuilding, Shula has relatively few question marks on his team. The Dolphins finished strongly in 1981, only a loss to San Diego keeping them from advancing further in the playoffs. And their success came despite extensive use of rookies and young veterans.

The position of most controversy is quarterback, where David Woodley and veteran Don Strock are competing again for the starting spot. Woodley is to start tonight, although Strock played better at the end of last season.

A few Miami rookies, however, have a chance to contribute. Receiver Mark Duper, a No. 2 pick from Northwestern Louisiana, has outstanding speed, although he has been slowed with a sore ankle. The Redskins rated him slightly ahead of Powell in the draft and he probably would have been taken by Washington if available.

The No. 1 pick, Roy Foster of USC, has been moved from guard to tackle, where he is a reserve. Yale's Rich Diana and Jackson State's Larry Cowan, running backs, have been impressive so far.

Shula also has some problems. Kicker Uwe von Schamann is injured, as is receiver Nat Moore, cornerbacks Ed Taylor and Gerald Small and fullback Woody Bennett. Free agents Hans Nielsen and Jorge Portela are doing the place kicking for now. For insurance, the Dolphins have been experimenting with halfback Tommy Vigorito at wide receiver.