Swirdfish wrote: may I ask what should I expect. ... but I'm more interested in the procedure itself and its effects afterwards.
If you are having a procedure to remove a tumor in the rectum, then the effects after the procedure will likely fall in the area they call LARS (Low Anterior Resection Syndrome). The effects of LARS are well known and are summarized here:
Low Anterior Resection Syndrome
http://colonrectalsurg.wustl.edu/en/Patient-Care/Low-Anterior-Resection-Syndrome
In your Tuesday meeting with the surgeon, I would recommend bringing up the question of LARS and to ask what he intends to do in order to minimize your LARS Syndrome effect.
As the name implies, LARS problems originate in the surgery itself and are related to what was removed or damaged there in the rectum when the malignant tumor was removed.
There are known surgical procedures for minimizing these problems, such as Sphincter Sparing Operations. I think it would be worthwhile for you to discuss with your surgeon the various approaches he plans to take to minimize the necessary and collateral damage to the important structures in the lower rectal area (i.e., to the sphincter muscles, to the autonomic nerve sheaths, the feedback nerves that connect the sphincter with the rectum, the various uro-genital structures in the area -- seminal vesicles, genital and urinary nerve connections -- etc.) I think you should also ask him exactly where your tumor is located and what is the prognosis for successful removal of a tumor in that particular location.
Images "A", "B", and "C" below show where the incisions would be made to do a Total Mesorectal Excision (TME) for a tumor located in the mid-rectum area for a female patient.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4057635/bin/11605_2014_2528_Fig1_HTML.jpg
You could ask your surgeon where your tumor is located and where (how low) he would intend to make the lower cut. This is what makes a big difference in the range of bowel management problems that you will eventually have to cope with from LARS: Lower cuts made somewhere in the upper two-thirds of the rectum are not as devastating as lower cuts made somewhere in the lower third of the rectum. This is why it is so important to know exactly where your tumor is located and from there exactly where the surgeon intends to make the lower cut. The lower the cut, the worse the eventual bowel management symptoms