As part of a preparation for the movie, Joaquin Phoenix trained for a month at a fire academy and spent another month with the men of Baltimore's Truck 10. He became an "honorary member" and had the same tattoo as the men of the company, a bumble bee wearing a fire helmet with an axe.
The firefighters Joaquin Phoenix trained with nicknamed him "Hollywood". Only after Phoenix earned their respect, they started calling him by his actual name.
The filming of the big warehouse scene caused something of a panic in Baltimore. The fire was visible from I-95 and the Baltimore Beltway, so Good Samaritans kept calling the fire department to report it. So many people called that the fire department ended up calling radio stations to confess that the scene was being filmed and to request that people stop tying up the emergency lines.
Joaquin Phoenix was nearly seriously burned in the scene where the factory floor collapses. If you slow down the DVD as he is sliding down the floor you will see a ball of fire caught between the floor and his chest. Luckily the crew awaited him with a fire extinguisher as he fell to pads below.
The scene in which John Travolta talks to Joaquin Phoenix while drinking liquor was improvised by Travolta. The scene originally had Mike Kennedy talking straight to Jack Morrison, but Travolta thought it would be funny to start drinking right in the middle of it.