Roger Taine is a salesman.
An infantry soldier during the war, Taine became an officer as a reward for valiant service and for having an uncommonly cool head on his shoulders during combat. As an officer, he quickly rose up the ranks to reach that of Lieutenant-Colonel. Once the war was over, he left the service and got a 'normal' job as a salesman for a large firm north of London, content to leave behind the danger and the medals.
Taine considers himself an extremely normal man, happily married with two small sons, working a normal, unexciting job but enjoying the opportunity to spend a good deal of time outdoors. There is undoubtedly, however, a streak of the adventurer in him that prompted his actions to get involved in things far outside his 'normal' life.
Taine's cool head, mentioned above, keeps him from getting too excited in any circumstance but that occasional desire for something different is what would cause him to peek through an open door simply because it is open and should not be. It is what propels him to act in a way most people would not. Though he is hardly an expert with guns or hand-to-hand combat, he is smart, resourceful, and unwilling to step back from a fight just because it might be prudent.
His participation with the British Secret Service comes about accidentally but he doesn't flinch when he is approached nor does he balk at the job they ask of him.
He indicates he is happy to go back to his normal life at the end of the first adventure but when called upon again a little later, he does not show the least bit of hesitation; the pull of the adventure had been too strong.