NOTE: This article is about the incarnation of Sideways from the Transformers live-action film series. The mainstream version can be found here: Sideways (Unicron Trilogy).
Sideways is a simple courier who avoided combat at all costs. He tried to always remain in the shadow of larger Decepticons, where the Autobots might not notice him. On Earth, he's keeping much the same practice. He teamed up with Demolishor early on, and hopes that if the Autobots find them, the big Decepticon can keep him safe.
Transformers: Revenge of the Fallen
NEST and the Autobots pursue Sideways and Demolishor through Shanghai. While Optimus and Ironhide are busy dealing with Demolishor, Sideways figures that he can escape now that they are sidetracked. But Arcee, Skids, Mudflap, and Sideswipe realize that he's going to get out of the toxic spill cover zone and they go after him. Sideways manages to evade all of them except Sideswipe, who leaps above him and slices him in half.
Transformers: The Ride
Sideways appears early in the Universal Studios theme park attraction, Transformers: The Ride. After Bumblebee grips Ravage who stole the AllSpark shard and ripped Ravage apart, EVAC was able to grab the shard with his hand, but then Sideways grabbed EVAC's hand with his left hand for the shard. Bumblebee then punched Sideways' left arm off, and proceeded to pound Sideways to death, allowing EVAC (carrying the passengers) to the remainder of the ride.
Originally, Barricade was meant to be the Decepticon that accompanied Demolisher, but instead it was changed to Sideways. It should be noted that Sideways has nearly the exact same appearance as Barricade.
According to a version of Ask Vector Prime run by Sideways himself, Sideways was rebuilt by Scalpel following his bisection in Shanghai. This provides logical evidence for his appearance in Dark of the Moon.
However, one can say that the Audi R8 Decepticon in that film is just another 'bot of the same body-type. After all, Sideways' model has been used for several other live-action Decepticons: Wreckloose, Dead End, and Override.