Discrete lamp dimmer circuit: how to choose component values?
This circuit triggers the triac when the voltage across the switch, after going through the RC circuit made from the rheostat and the 100nF capacitor, exceeds  the diac trigger voltage. After that, the triac triggers and the voltage drops to a volt or two, so the power dissipated by the rheostat is low. It can have mains voltage across it, but only when it is turned to maximum. 

Because the triac turning on "resets" the capacitor, this circuit has an annoying snap-on hysteresis effect around where it first turns on (minimum brightness). 

The rheostat resistance is chosen high enough that it doesn't overheat with mains voltage across it. Say 0.25W is acceptable, then R \$\ge \frac{230^2}{0.25} \$~= 200K. 50K would dissipate more than 1W, and would require a 1uF capacitor, neither of which is desirable, but not impossible if the pot is suitably rated. 

The capacitor is chosen to give an appropriate delay for 1/2 cycle of the mains. The math gets a bit messy because it's a sine wave input, but 100nF is about right for a 500K pot on 230V. Illustration from GE SCR manual.