How to if casting directors cast from showreels
Casting Directors will never cast from showreels because it is not them who have the final decision. Casting Directors put out the breakdowns and call suitable actors in to see the director and/or producer. The casting director will advise or assist in the process but ultimatley it will be up to the director/producer to make that decision on wether to cast you from your showreel or audition.
As a director I have used external casters to collate the candidates together and sometimes I will look over the CV's and headshots with casting director to make a decision on who to call in. Ultimatley I will make the decision on who to cast. Showreels give more insight into the actor but only if its needed.
As an actor I was cast form a showreel once and this was only because it was quite a last minute project and expenses only.
It is very hard to make an informed decision on an actor form a showreel because they can prove to be very different in person.
For some major productions, the process of selecting actors for sometimes hundreds of parts may often require specialized staff. While the last word remains with the people in artistic and production charge, a Casting Director or "CD" (and sometimes the Casting Associate) are in charge of most of the daily work involved in this process during pre-production. A Casting Director is sometimes assisted by a Casting Associate; productions with large numbers of extras may have their own Extras Casting Director.
The "CD" remains as a liaison between director, actors and their agents/managers and the studio/production company/theatre company to get the characters in the script cast.
So in summary it would be impossible to cast from showreels, especially on larger projects since the final decision goes through many different channels.