Who can the play the roles of Product Owner and ScrumMaster? And do they need to be full-time?
The ScrumMaster
Can the ScrumMaster perform the role of Product Owner?
No. There is a potential for a conflict of interest. One of the duties of the ScrumMaster is to protect the team from over-committing to an over-eager Product Owner. If I am performing both roles, we’ve got a problem.
Can a developer be the ScrumMaster?
This was generally what happened when I first started using Scrum. One of the dev team would act as ScrumMaster for a Sprint, then pass on the duties to another dev for the next Sprint. It wasn’t a disaster, and worked well enough for a small development team, but I’ve seen it work better when there is a dedicated ScrumMaster. A dev is a specialist in, well, development, so does not necessarily hold the skills needed to be a good ScrumMaster. Larger organisations usually benefit from stand-alone ScrumMasters because they will often take on coaching responsibilities too, working with the broader business.
Does the ScrumMaster need to be full-time?
Yes. There needs to be a ScrumMaster available when the development team is working. Ideally they sit with the team, but they should at least be contactable by phone, email, instant messenger, etc. The team is not going to work as well if their ScrumMaster only works 3 days a week.
Product Owner
Can the Product Owner be a dev?
This isn’t as bad an idea as the ScrumMaster being a PO, but it is normally quite a hard situation: the PO not only needs to form a relationship with stakeholders (i.e. the business), but also have the position/status within the organisation to make tough decisions which stick. Would your stakeholders be happy with one of the devs deciding on the prioritisation of requirements?
Does the Product Owner need to be full-time?
Yes. The PO needs to be available when the development team is working. Ideally they sit with the team, but they should at least be contactable by phone, email, instant messenger, etc. A Product Owner who only works 3 days a week is going to be problematic.
There are bound to be exceptions. I’m not saying it’s impossible for the above to work (for example, a 2-person company), just that I’d not recommend it.