Every other article in this series has assumed a clean separation between the expert witness role and the role of advocating a party's case. That separation is not incidental — it is close to the foundation the entire concept of expert evidence rests on. This piece looks at why the two roles are fundamentally incompatible, the narrow circumstances in which a dual role is sometimes tolerated, and why construction and engineering experts in the GCC's principal fora should approach any blurring of the two with real caution.
Two roles, two different jobs
An expert witness's job is to give the tribunal an independent, technically grounded opinion, whether or not that opinion favours the party who instructed them. An advocate's job is precisely the opposite in orientation: to present the client's properly arguable case as persuasively as the evidence and facts allow, doing for the client what the client would do for themselves if they had the skill to do it. These are not simply different emphases on the same underlying task. They require different mindsets, different standards of candour, and — critically — different duties. The advocate's duty runs to the client, within the bounds of the tribunal's rules. The expert's duty runs to the tribunal, and overrides any obligation to the client.
Asking one person to occupy both roles on the same matter asks them to hold two competing loyalties simultaneously, and to keep the tribunal, and every other party, confident that the two have not bled into each other. That is a genuinely difficult thing to do convincingly, which is why the professional standards governing expert evidence treat the dual role as an exception requiring justification, not a routine option.
Where a dual role is sometimes tolerated
Some lower-tier tribunals and dispute resolution fora, in limited circumstances, permit a party representative to act in both capacities — as advocate and as expert witness — typically where the disproportionality of retaining two separate professionals would otherwise limit a party's practical access to justice. Even where this is permitted, it is treated as an exception applicable to specific, usually lower-value or lower-complexity proceedings, not as a general model. In DIAC arbitration, before the DIFC Courts, or in arbitrateAD proceedings — the fora handling the significant construction and infrastructure disputes this series is principally concerned with — a dual role of this kind is rarely appropriate and rarely permitted, and should not be assumed available without direct confirmation of the relevant tribunal's rules.
What must happen before a dual role is even considered
Where a dual role is genuinely under consideration — most plausibly in a smaller, lower-tier proceeding — the professional carrying it should, before agreeing to proceed:
- Confirm that neither the rules nor the customs of the specific tribunal prohibit acting in both capacities
- Assess honestly whether other factors make the dual role appropriate, including whether the cost of retaining two separate professionals is genuinely disproportionate to what is at stake
- Communicate clearly, in writing, to the client the likely advantages and disadvantages of proceeding this way — including the probable impact on the weight the tribunal gives to the expert evidence, and any impact on the advocacy submissions themselves
- Only proceed where the client, having been given that information, instructs accordingly and the tribunal permits it
Even having cleared all of that, the professional acting in a dual role must advise the tribunal of that status and clearly distinguish between the two roles at all times — in written submissions and in any oral proceedings — so the tribunal always knows which hat is being worn at any given moment.
Why blurring the two roles damages both
Even in permitted contexts, the practical risk of a dual role is significant. A tribunal assessing the weight to give to expert evidence delivered by someone who has also argued the client's case directly is likely to discount that evidence, consciously or not, precisely because the independence the expert role depends on has been visibly compromised by the advocacy role sitting alongside it. The advocacy submissions, in turn, can suffer if delivered by someone the tribunal has also had to assess as a witness — the professional's credibility in one role affecting perceptions in the other.
For a contractor or engineering business managing costs, the apparent efficiency of using one professional for both roles can therefore be a false economy: expert evidence that is discounted, or an advocacy submission weakened by association, may cost the client considerably more in outcome terms than the saving achieved by not retaining two separate professionals.
Conditional fees and the dual role
Where a dual role is genuinely permitted, fee arrangements that would be entirely inappropriate for an expert witness — conditional or success-based fees — may sometimes be tolerated for the advocacy component of the engagement, on the basis that they attach properly to the advocacy role rather than the expert role. This distinction, and why conditional fees are treated so differently depending on which role is in play, is the subject of the next article in this series.
The safer default
For any construction or engineering dispute proceeding through DIAC, the DIFC Courts, arbitrateAD, or comparable regional institutions, the safer and more common default is to keep the two roles entirely separate, retained by different professionals, with clear and unambiguous lines of responsibility. Where cost proportionality is a genuine concern, that concern is usually better addressed through the scope and structure of the expert's engagement than through collapsing two fundamentally different roles into one person.
Part 15 examines fee arrangements for expert witnesses directly, and why contingency or success-based fees are considered fundamentally incompatible with the expert witness role.