Interim management is a temporary injection of skills into an organisation.
The interim will typically work within an organisation to manage a transition, change or crisis. Because organisations do not need these skills at all times, interim management tends to fall outside IR35. Some interim roles lend themselves to fixed-price contracts. In this situation, the interim is paid to achieve pre-defined milestones, goals and benefits.
Freelancers take on specific tasks, most often for a fixed price. The boundaries of the task are clearer and the engagement is almost certainly outside IR35.
In project management, an interim enters an organisation that does not need the skills of a project manager on a permanent or fixed term basis. They spend most of their working day within the organisation but keep a distance to provide a ‘fresh pair of eyes’. Interim contracts generally last between three and nine months.
On the other hand, many freelancers stand apart from the organisation they serve. Unlike the interim, they are likely to pop in and out of the business and might work with several clients at once. A freelance activity might last an hour or hundreds of hours over several months.
To be honest, the difference is more audience related. I find larger companies prefer interim and many SMEs are comfortable with freelance. There are situations when both are synonymous with self-employment or contractor.