Note: Does not strictly adhere to the texts.

Balaram was never the mastermind.
The pranks were planned and, mostly, pulled by Krishna.
Much as he hated getting into trouble, he didn't quite object to taking the blame when his brother was cornered. This, sort of became a tradition, and a time was reached when he did not even bother consulting his brother on what the trouble was all about.
He was, of course, calmer of the two.
Calmer, if you do not count that time he broke that Asur's neck when he came for Krishna and the time he was about to murder their treasurer when he accused Krishna of some real theft, and that time when he chased Krishna across the better part of the forest after, that mischievous brother of his persuaded him to enter a garden filled with crazy girls anticipating Krishna's arrival, and vanished on the spot.
He was a naturally sweet person with a splendid sense of humor, but you wouldn't want to cross him when his spirits were unnaturally high, or low. Though he had a rash temper, anyone who knew him would agree that he was a totally awesome man to be with.
However, despite succumbing to many tricks over the years, he was never able to detect when Krishna might be up to some trouble.
Once, Krishna had virtually dragged him from his bed to the peak of Raivatak, the hillock on the other side of the city to meet his WIFE. Another time, he left for an alien kingdom in the dead of the night, with no guard other than his charioteer and some Brahman, leaving nothing but a short note citing his destination.
What annoyed him the most was that every time he concluded something to be the limit, his talented brother managed to get himself into a greater problem. What's more, Krishna did not even think they were troubles.
"WHY on earth did you not tell me BEFORE I promised her hand to Duryodhan?" he had asked his brother, after their cousin Arjun kidnapped their sister Subhadra from the temple on the eve of her marriage. Krishna had merely smiled.
Sometimes, by the seashore he thought he felt something about knowing more than what he had been told, but the flashes ebbed before the twirling waves.
He never bothered much with them. He was a well-built, accomplished man with an untarnished reputation for fairness, loved by his wife, siblings, parents and children, and he did not think he wanted more.
He was happy being the way he was.