Leathers must be fitted closely so the armor stays in the correct places if you take a fall. This is also important to reduce the probability that the leather will bunch up and catch if sliding. If it catches, then it will promote tumbling in a fall.
New leathers must be a bit on the tight side. They will stretch with use to fit your body and any armor within them or worn under them.
It is expected that the leather WILL restrict movement of your limbs. You will probably not be able to put your arms above your head, and your elbows may not bend all the way. These are non issues on the bike, however -- and you really don't want your arms to be able flail above your head in a tumble.
I wear a VelocityGear Juggernaut armored shirt with back and chest protectors under my leathers. The armor has been removed from the leathers so they fit properly. I believe the Juggernaut is better armor than can be found in most suits, and I can wear it under almost any properly-sized motorcycle jacket or suit.
It is important that any leathers you buy be made for motorcycle use. Fashion leather may not have the strength, abrasion resistance, or seam construction that will come into play in a fall. Same goes for boots and gloves.
Textile suits may not be able to withstand more than one fall to the tarmac before requiring replacement. Also, many textile suits do not hold the armor securely enough in position. I have an Icon TiMax jacket and pants set that looks great, but I feel unprotected in it even with the armored shirt beneath it. The attachment between the jacket and pants is very insecure looking, consisting of three snap-fastened straps. This may allow separation at the waist, resulting in road rash. The suit material is said to be substantial enough, but I believe leather is more so. I have not "road tested" this outfit in a fall.
Two-piece leathers with full-circumference zipper at the waist are very convenient for road use, and can be used at most tracks. Many are not perforated or well ventilated, so they can be hot. One-piece leathers are less convenient for street use, but look good and are very functional. Many are perforated for cooling, and are quite comfortable up to about 95 degrees when moving. Liner garments can be worn for lower temperature use. Almost any leather (and many textiles) will be hot when stopped. One piece leathers are also very difficult to get into and out of. Once in, they are very comfortable.
Underwear is important to be able to slide the suit on and off as mentioned above by another poster. Cotton underwear is not good, it gets soaked and doesn't dry easily. Polypropylene or polyester is better. Bicycling shorts add a little extra padding where needed for long jaunts. Polar fleece or polar pile is good for cooler weather.
A tip about riding in high temperatures: Maintain hydration well beyond what you think you need. Drink water to the extent that you must urinate at every stop. When you're "in the wind" your fluids leave your body quickly. If you get even mildly dehydrated, your mental ability will suffer and you'll be prone to panic and bad judgement.
OK, to the answer: I never ride without wearing full leathers. Two piece for commuting, one piece for the longer rides in the mountains. Once in them, I forget they are on me. They feel good, and I feel protected. I have taken a fall with a Fieldsheer two-piece leather suit, gloves, boots, helmet, and Juggernaut armor without any road rash and with relatively minor tendon/ligament/muscular damage that healed up in a few weeks on its own. No broken bones. (Your results may vary.)
See this video for some help in fitting a leather one-piece suit:
YouTube - Leather fit
...but these guys are afraid to help one another get out of their leathers. Must be a guy thing along the lines of never asking for directions when lost (I can identify with that):
YouTube - The Leather Dance
Go for it...and then post a picture!
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Leathers?
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