First consideration for an on-off valve is to get enough force to cause the flow to stop.  That is dependent on the valve.  There are more variables than anybody wants to read through, but those touched on earlier in the thread are valve type, size, seat material, probably shutoff pressure, process fluid lubricity, type of packing.... But it must be known so that the actuator can be specified.
The force required may change in service due to corrosion, product buildup, deterioration of the seats, "set" friction, and/or other variables.  This is why there is a safety factor. 
The actuator characteristics should be matched to the valve's torque characteristic.  Butterfly valves have high breakout torque and a high dynamic torque at a near-open position.  This corresponds nicely to the torque available from a scotch-yoke actuator.  Ball vales have a breakout torque and the run torque drops off to 60-80% of the breakout torque, mostly because the pressure imbalance on the ball goes away after the ball opens.  Rack and pinion actuators cope well with this.  
But, the thread was not limited to rotary valves. 
Gate valves are frequently if not predominantly used for isolation. R&P and scotch yoke actuators are not useful for rising stem valves.  Gate valves need GOBS of force to force the discs into the wedge seating members used for shutoff (Even if the disc is the parallel-seat double-disc type) and after the valve cools down and grabs the disc they need as much or more force to yank the disc out of its seated position. after the end 1/16", the gate valve then only has packing friction and the weight of the components so it is relatively easy to move.  To handle a gate valve you need a big electric (e.g.: Limitorque) with hammer-blow feature, or a huge oversized pneumatic cylinder, or a hydraulic cylinder.

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