Do departments overlap on some projects?

Yes they do, often we work together with other teams to provide clients with a range of services. For example, my team provides financial due diligence and often tax teams carry out tax due diligence.

I echo Frances' comments, I often work on large tax audits which involves a lot of cross-department working between the main audit team and our specialist team. This applies to other specialist areas such as actuarial etc

Similarly in Assurance we regularly work with Tax, Pensions, and Valuations on our audits where we don't have the specialist knowledge.

There are many business areas that cross over, and even sub-business areas cross over a great deal. E.g. within tax, corporate tax will often work very closely with indirect tax (VAT) and Transfer pricing teams. There are always collaborations going on everyday. My team work closely with individuals and their personal tax obligations, we may need to work with the private tax team if the individual is a high net worth individual.