In one of my stories, I have a bipedal walker. Basically a copy of a Star Wars walker, but smaller and armed with M16s and grenade launchers. After a few minutes of research and using my common sense, I realized that such a walker would be impractical and nearly impossible. The main problem would be the CoG (Center of Gravity/Mass/Balance), which would probably be in the center of the 'head'. However, when taking a step, the narrow rectangle between the two legs where the CoG can be to avoid falling over becomes a small square over the foot. Here's an excerpt from the first time this walker was introduced to the story:
The warehouse doors were suddenly blown off, and inside was a huge bipedal mech. On top of the mech was (insert evil scientist name here), laughing that weird evil person laugh. "You fools! You have wandered right in front of my MMD, Mech of Mass Destruction! HAHAHAHAHA! Utilizing this stolen miniature nuclear generator prototype, I now have the unlimited power needed to power this mech!" He pushed a lever in front of him, and the mech fell over on its side, tearing open the side of the warehouse. "No! I told Gerald to fix that!" When the dust settled, evil scientist man had disappeared, and the nuclear generator prototype along with him.
What I think evil scientist man needs to do is either: make a way to change the CoG between steps, or have some really big feet pads. For the first option, my rather limited knowledge is imagining a giant servo on top that rotates a weight side to side each step, but this would make it much wider and it would be an obvious attack point. The second option would probably be easier, but would require smoother terrain to stay upright.
I want this solution to be 100% possible with today's technology, and preferably not something on the exterior of the mech.