Who strikes and where? Who provides the weapons? Who provides the intelligence? And who provides the cash?
The international conference in Paris on Monday that gathers some 20 countries from the anti-Islamic State coalition will seek to divide up the roles between nations with often diverging interests.
"This conference will allow everyone to be much more precise about what they can do or are willing to do," said a French diplomatic source, who did not wish to be named.
However, any decisions taken at the conference, jointly hosted by French President Francois Hollande and Iraqi President Fuad Masum, will not necessarily be made public, the source stressed.
"We're not going to say who is going to carry out air strikes. Or when they might happen."
Most countries will be represented by their foreign ministers and the conference will be the last stop of a marathon tour by US Secretary of State John Kerry, who has been criss-crossing the region to build as broad a coalition as possible against the IS militants.
But the run-up to the conference has been marked by thorny questions surrounding the position of Russia -- embroiled in a diplomatic row with the West over Ukraine -- and Iran, a major regional player but which has sought to link other issues to its cooperation against IS.
In the week leading up to the conference, the burning question was whether Iran would come or not, after France indicated Tehran might receive an invitation if it agreed not to link concessions over its sensitive nuclear programme.
However, Kerry slapped down the possibility, telling reporters it would "not be appropriate, given the many other issues that are on the table with respect to their engagement in Syria and elsewhere."
Tehran has offered help to Iraq in fighting the IS militants that have taken a swathe of land in the north of the country but has also backed the discredited Syrian regime of Bashar al-Assad.
A spurned Iran said Saturday that they didn't want to come anyway, saying the conference had a "selective guest list" and was "just for show."
Another tricky country to deal with is Turkey, which shares borders with Syria and Iraq and remains a close US ally with the NATO alliance, but is reluctant to be pulled too deeply into the conflict given that IS militants hold 49 Turkish citizens, including children and diplomats.
"We have to be sure that the left hand doesn't contradict what the right hand is doing" in the coalition, said the French diplomat.
Iraq 'heart of the problem'
French foreign ministry spokesman Romain Nadal said: "We are looking to bring together the aims and initiatives" on the military, humanitarian and financial fronts.
Nadal said there would be concrete decisions announced at the conference and a plan of action drawn up.
But another stumbling block is Syria -- of which IS controls roughly a quarter, compared to approximately 40 percent of Iraq.
Washington has voiced a willingness to strike at IS in Syria without the backing of Assad, while others are more hesitant.
In London, while Prime Minister David Cameron has said he will not rule anything out, Foreign Minister Philip Hammond stressed Britain would not take part in strikes against IS in Syria, after parliament last year voted against taking military action in that country.
Britain's resolve may well have been stiffened however, by the execution of one of its citizens, aid worker David Haines, by IS militants.
France is unwilling to take part in action that is not backed by the UN Security Council, which is unlikely to approve military strikes given Russian and Chinese opposition.
Hollande has repeatedly stressed that there is no question of working with Assad to battle militants in Syria and French diplomats seemed keen to focus on one problem at a time.
"If we want this conference to be useful, we should not mix up the problem areas. The heart of the problem at the moment is Iraq."