Former Montreal mayor Michael Applebaum, who is charged with various fraud-related counts, arrives at the courthouse, Monday, November 14, 2016 in Montreal. THE CANADIAN PRESS/Paul Chiasson

MONTREAL - The Crown's key witness at ex-Montreal mayor Michael Applebaum's corruption trial says he was often conflicted about working with police against his former boss and that it took a mental and physical toll.

But former Applebaum aide Hugo Tremblay testified Wednesday he was determined to live up to his end of the bargain after agreeing to work with anti-corruption police.

As the defence continued its cross-examination of Tremblay, the witness explained having to cope with many emotions and said he even went three times to a psychologist referred by police.

Tremblay said he was sad police were disappointed the results of the last of the wiretap attempts against Applebaum in 2013 didn't go as well as they'd hoped.

That last one was done over the phone and Tremblay had told police it was too scripted to ever work with an ever-cautious Applebaum.

Defence lawyer Pierre Teasdale questioned Tremblay about why he plowed ahead even as he dealt with health issues, a conflict of values and extreme stress.

Tremblay said he'd signed an agreement to help police and was determined to follow through.

"The human spirit is complex," he added.

Applebaum has pleaded not guilty to all 14 corruption-related charges, including fraud against the government and breach of trust. They stem from crimes alleged to have occurred between 2007 and 2010 when he was mayor of Montreal's largest borough.

The Crown alleges Applebaum, who went on to become interim Montreal mayor in 2012-2013, accepted cash via Tremblay in exchange for favours given to local real-estate developers and engineering firms.

Teasdale hammered away at Tremblay's close relationship with a Quebec provincial police detective after agreeing to turn. He questioned the witness about a large number of text messages between the two men as Tremblay wore a wiretap to help police build a case against Applebaum.

At one point, Tremblay complained in a text message about the pressure he was feeling, prompting the investigator to reply: "You're part of the team."

Tremblay, on the stand for a third day, testified he struggled mightily with what he was doing and that speaking regularly to an investigator helped him cope.

Tremblay said he was too caught up in the case and didn't initially agree to a polygraph with police, but added Wednesday he'd gladly take one to prove his version of events.

"I'm ready to take a polygraph today, without any problem," Tremblay said.

The trial, which began Monday, is expected to last a total of at least two weeks.

News from © The Canadian Press, 2016