Calculate percentage of similar values in pandas dataframe

You could divide by the sum along the first axis and then cast to string and add %:

out = (df.set_index('Speaker')['Script'].str.findall('|'.join(L))
         .str.join('|')
         .str.get_dummies()
         .sum(level=0))

---

(out/out.sum(0)[:,None]).mul(100).astype(int).astype(str).add('%')

            a     b    c
Speaker                  
Speaker1   50%   25%  25%
Speaker2  100%    0%   0%
Speaker3    0%  100%   0%

(df.set_index('Speaker')['Script'].str.extractall(f'({"|".join(L)})')
   .groupby('Speaker')[0].value_counts(normalize=True)
   .unstack(fill_value=0)
)

Output:

0            a     b     c
Speaker                   
Speaker 1  0.5  0.25  0.25
Speaker 2  1.0  0.00  0.00
Speaker 3  0.0  1.00  0.00

Starting from your original dataframe, if you want % and not grouped sum of dummies , you can change the entire script like below:

m = df.set_index('Speaker')['Script'].str.findall('|'.join(L)) #creates a list of matches
m = m.explode().reset_index() #explode to a series 
final = pd.crosstab(m['Speaker'],m['Script'],normalize='index').mul(100) # percentage pivot

---

Script         a      b     c
Speaker                      
Speaker 1   50.0   25.0  25.0
Speaker 2  100.0    0.0   0.0
Speaker 3    0.0  100.0   0.0

If you dont want the percentage just use:

pd.crosstab(m['Speaker'],m['Script'])

---

Script     a  b  c
Speaker           
Speaker 1  2  1  1
Speaker 2  2  0  0
Speaker 3  0  1  0

Note: this uses pandas 0.25+ as version