The first debate of the campaign was never meant to fall within the election period, but the early call means it comes only five days into the writ period.

And while all the parties say their leaders have been working on debate prep for weeks, if not months, only Conservative Leader Stephen Harper and Green Party Leader Elizabeth May have done a national leaders' debate before.

For NDP Leader Tom Mulcair and Liberal Leader Justin Trudeau, it will be their first.

With that in mind, here are five things to watch for during tonight's debate.

1. The leaders' comfort levels

The leaders will want to look relaxed, at ease and confident to give the image of someone who knows his or her material. They'll also want to project a combination of warmth and competence, said David McLaughlin, the former chief of staff to Brian Mulroney.

"They have to show that they are friendly and comfortable enough, that likability factor," he said, "with the sense of they actually are competent enough to know the files and run the country well."

Green Party Leader Elizabeth May's last federal leaders' debate was in 2008. (Tom Hanson/Canadian Press)

Trudeau and May tend to come off as friendly, while Harper and Mulcair can be seen as stiff.

And while it's important for leaders to get out the message that their policy is better than that of the others, they have to do that without turning off voters by sounding nasty. McLaughlin said that's done through scripting and preparation.

"You find a way that makes certain that you find the right tone and the right words that makes the point without exactly coming across as mean-spirited," he said.

"You're talking to people in their living rooms."

2. How Mulcair and Trudeau interact

More than two months away from voting day, the NDP has a narrow lead in the polls, with the Conservatives second and the Liberals third. But a three-way race, like the current one, can result in vote-splitting that leads to unexpected results.

NDP Leader Tom Mulcair responds to a question during an NDP leadership debate in January 2012. (Tim Krochak/Canadian Press)

For that reason, the NDP and Liberals are battling each other to draw the centre and centre-left votes, and Mulcair and Trudeau need to contrast themselves with each other as much as they do with Harper.

"The Trudeau-Mulcair piece is the one that I'm going to be most interested in, because it's going to really show the battle for the left," McLaughlin said.

"There's two referendums happening here. A referendum on Mr. Harper: Do we keep him [as prime minister] or do we dump him? The other referendum is who's going to take the mantle of the left … to be that actual plausible alternative. Six months ago, it was Mr. Trudeau. Now it's Mr. Mulcair."

3. The non-verbal signals

The leaders can telegraph messages through everything from their clothes to their facial expressions. McLaughlin recommends watching body language in particular to see which leader at any particular time is trying to siphon votes from another.

Conservative Party Leader Stephen Harper answers a question during the English-language federal election debate in Ottawa in April 2011. (Chris Wattie/Canadian Press)

"That [physical] pivot that happens in a debate typically means that the debate is shifting and that the leader who pivots realizes, 'I've got to engage this opponent on something,'" he said.

"That body language and the way that they engage tells you an awful lot about one thing in particular: whose votes they're going after."

It's also worth watching to see whether any leader is ignored: Harper and Mulcair may want to ignore Trudeau to suggest he's the kid trying to play with the adults, or the other leaders might ignore May because, with the least number of seats in the House before Parliament was dissolved for the election, she may be perceived as the least threatening.

4. The ridings they mention during the debate

If you enjoy the strategy behind campaigns, you'll want to pay attention to which examples the leaders bring up to illustrate their points. They'll link back to the party's target ridings. Trudeau, for example, who has his sights set on winning more seats in the Vancouver area, may choose to talk about a middle-class family in that city (expect the terms "middle class" and "hard-working families" to come up repeatedly during the debate).

"They go into these debates not just with their messages in mind, but also with their voting pools in mind," McLaughlin said.

"They know who they need to appeal to."

Then leadership contestant Justin Trudeau speaks during one of the Liberal Party's debates in 2013. (Andrew Vaughan/Canadian Press)

5. How the format affects the debate

The format of the debate tonight will be different from the debates in recent elections. It's the first not to be organized by the broadcast consortium, of which CBC is a member, and could be the only English-language debate of the 2015 election campaign to have all four leaders of the major national parties.

The format allows for one-on-one interaction between debate moderator Paul Wells, Maclean's magazine's political editor, and each leader, which means the leaders have to prepare for questions from a seasoned journalist on top of getting set to take on their rivals.

"The moderator can set the tone for a debate and can cut them off and can bring them back down to earth in some way," McLaughlin said.

"If you're consistently being cut off by the moderator or being chastised in some way, that's going to look bad to voters."

Follow the Maclean's federal leaders' debate with CBCnews.ca. Our pre-debate live chat begins at 6:30 p.m. ET, and we'll follow reaction to the debate live between 8 and 10 p.m. ET. Then share your thoughts on how the leaders fared in our post-debate discussion. Find our live blog and analysis at cbcnews.ca/canadavotes.

You can watch the Maclean's debate live at 8 p.m. ET in English on City-TV, in English and French on CPAC and in Italian, Punjabi, Mandarin and Cantonese on OMNI TV, and on YouTube.