Diamondbacks manager Kirk Gibson has already seen a much more expressive side to his new right-hander.
Cahill makes his NL debut Tuesday night against the San Diego Padres when the visiting Diamondbacks look to open 4-0 for the first time.
A 2006 second-round pick out of Vista High School in Oceanside, Calif. - 40 miles north of San Diego - Cahill went from an 18-game winner in 2010 to a tradable commodity after going 12-14 with a 4.16 ERA last season for Oakland.
Arizona acquired him in December in a deal that general manager Kevin Towers felt would give the defending NL West champions another talented starter to slot alongside Ian Kennedy, Daniel Hudson, Josh Collmenter and Joe Saunders.
Forst said that Cahill, who will earn $3.5 million in 2012 - $3 million more than last season - understood the economics of the deal and why the rebuilding A's pulled the trigger.
"Trevor never gets too high or too low with anything," Forst said.
Gibson would probably disagree with that. With Cahill set to make his Diamondbacks debut, Gibson hopes the 24-year-old can keep his composure when he steps on the mound at Petco Park.
"He gets pretty hyped up," Gibson told the Diamondbacks' official website. "We talked about it and it's something we're going to have to work on. You wouldn't think that because he's a really laid-back guy."
Cahill, who had a rocky spring with a 4.86 ERA in 16 2/3 innings, has never pitched against San Diego.
He'll face a Padres team that avoided its first 0-4 start since 1994 with Sunday's 8-4 victory over the Los Angeles Dodgers.
"We really didn't play how we're capable of playing or how we wanted to play, but I think we averted disaster," said third baseman Chase Headley, who snapped an 0-for-12 start to the season with his second career grand slam.
"If you lose the first four, you really put yourself behind the 8-ball. So it's good to get that first win out of the way, and hopefully we can refocus and play better going forward."
The Padres would like to see better control from Edinson Volquez (0-1, 3.60 ERA) on Tuesday after the newcomer walked four and yielded three runs in five innings of Thursday's 5-3 season-opening loss to the Dodgers.
"The walks ... I can't explain it," manager Bud Black told the Padres' official website. "He just couldn't find the key pitch to get him through the fourth."
Volquez went 0-1 with a 5.19 ERA in three starts versus Arizona while with Cincinnati.
The Diamondbacks lost five of nine in San Diego in 2011, but they open their six-game trip with plenty of momentum after capping a three-game sweep of San Francisco with Sunday's 7-6 victory. Arizona matched a franchise record by rallying from six down to win.
The Diamondbacks led the majors with 48 come-from-behind wins in 2011.
"I think it brings back good memories for us," Gibson said.
Diamondbacks right fielder Justin Upton has plenty of those against the Padres after hitting seven homers in last season's series as Arizona won 11 of 18. It was the most home runs against San Diego in a season since San Francisco's Barry Bonds hit seven in 2002.
Upton, 2 for 9 with four strikeouts this season, has three hits - including two doubles - in six at-bats versus Volquez.