You got a lot of replies from a lot of different locations, and that is one key to the differences in opinion. I live just up the road from you in Colorado, and I assure you that being a DM in Colorado is different from being a DM in any location where people can routinely go out and do their OW dives nearby. In Colorado, if your shop is conducting OW dives in one of the few local sites available, you may well find yourself setting up materials ahead of time (such as a dive platform, if you use one) and watching for diver problems in low visibility. In the CW classes, you will find yourself mostly watching to make sure everyone is OK during the instruction. That is an excellent time to watch a skilled instructor at work and prepare for your future.
You may also get a chance to teach discover scuba and scuba review classes. These are great opportunities to hone your instructional skills.
Now that I have narrowed it down to the state, it will also vary from LDS to LDS. I just described how it was when I started as a DM, but it is different in that shop now. We do not use DMs to assist CW classes anymore. There are so many professionals available that we only use AI's for that. I had to get my AI in order to continue assisting classes, and I wisely chose not to get my full instructor at the time because we already had too many instructors, and I would only be allowed to work as an AI anyway.
Working for a time as an AI gave me more valuable time watching skilled instructors at work. When I finally started instructing myself, I had picked up a lot of good tips and had a good sense of how to get students through the full instructional sequence.
As for your situation, the owner of the shop is always dealing with some level of staffing insecurity. People come and go. When I was working on my DM, the shop was fully staffed and didn't need anyone. I got my DM and began to look around for work, and at that moment the shop decided that they needed another DM after all, and they offered me the position. It is now years later and all three people in my DM class, none of whom had any indication that they had a DM position in the offing when they took the class, are now working as instructors for the shop.
I would guess that when you take the class, you will be watched carefully for your potential. If the shop has true instructional integrity (and I believe every CO shop I know does have this quality), it will only want to hire people who will preserve or improve its reputation. You will not be expected to be perfect. You will, however, be expected to demonstrate a willingness to learn and (especially) the ability to work patiently and effectively with the most troubled students.