The Rev Ian Paisley favoured religious fundamentalists over his DUP deputy leader Peter Robinson during the first roundtable peace talks between the main parties in Northern Ireland, records disclosed.
He was impulsive and appeared to set impossible conditions in 1991, according to a note published by the Public Record Office of Northern Ireland (PRONI).
The Brooke/Mayhew negotiations excluded Sinn Fein, but were the first to have three strands: the relationships within Northern Ireland, between Northern Ireland and the Republic of Ireland, and between London and Dublin.
An NIO official wrote: "There have been very clear indications that he (Paisley) is very aware of his religious fundamentalists, who are against any political compromise, and instinctively favours them over the career politicians such as Robinson."
The note added that Mr Paisley was a clergyman before he was a politician and he would not want to offend his church followers.
"If it comes down to a stark choice Paisley is more likely to block political progress than damage his (Free Presbyterian) church.
"He was deeply affected by some serious conflicts within his church raised by fears that he was selling out at the talks.
"There are no clear signs at present that Paisley has the will to risk taking on his church followers and give them a clear lead to accept changes which they may find difficult to accept."
It said former UUP leader James Molyneaux provided some stability in their relationship.
"He seems to be the more cunning and steady operator and he also knows where he wants to end up."
The official record said Mr Paisley had the charisma and popular support which his unionist counterpart lacked, and suggested the British government work on the more volatile DUP leader to loosen up his UUP colleague.
"Paisley is also the key factor in selling any political package to the wider unionist community; more than Molyneaux he is the authentic voice of gut unionism."
It said a newspaper interview by Mr Paisley in 1991 appeared to set "impossible" conditions for a resumption of talks and took his deputy leader by surprise.
"Paisley's strategic aim, and indeed his attitude to the recent talks, remains obscure.
"It may be that he trims to suit the circumstances of the moment, or that he is conscious of the looming general election and wishes to keep his options open and avoid committing himself until the ground on which he has to do battle with the UUP becomes clear."