Birmingham City boss Steve Bruce has revealed how even a closed-doors friendly against Aston Villa can spill over.
The recent practice match was meant as a fitness exercise, but Birmingham manager Bruce revealed it was "kicking off everywhere".
Villa boss Martin O'Neill described it as "feisty".
Bruce said: "I know they're our fiercest rivals but Martin needed a few players fit, I needed a few fit, so we organised a game between ourselves - and it was a bit spicy, to say the least.
"It was incredible. I don't know who the referee was but he played his part - he let everything go. It was just what both of us wanted. It was a proper, no holds- barred game. Expect the same again on Sunday.
"Martin and I just stood there enjoying it. We got far more out of it than you could get out on the training pitch. You don't normally get that kind of game, but I've got the hugest respect for Martin - he's a football person."
O'Neill was just as enthusiastic about the game, which Villa won 4-3 at their Bodymoor Heath training ground, although Birmingham's fit-again Mikael Forssell gave a warning of the threat he poses with a hat-trick.
"It was lively stuff, quite strong really, and I don't think either of us expected it to get as feisty as it did," explained O'Neill.
"What it confirmed to me was this is not an ordinary game on Sunday. They're meaty affairs with plenty of atmosphere. It's a big derby, my first, but not the players' first. I just hope we keep a clear head and show some discipline.
"I don't want to end up as we did against Manchester United with nine players."
